Response of the EQCM to specific adsorption on gold electrodes: adsorption of pyridine and benzene

The electrochemical quartz crystal microbalance (EQCM) is used to study adsorption of pyridine and benzene on gold. The most important result observed is that the frequency increases when the organic molecule is adsorbed, namely, the total mass of the species rigidly adsorbed is reduced as a result of adsorption. This is consistent with the accepted idea that electrosorption is a replacement reaction, in which a number of water molecules are replaced from the interface for each organic molecule adsorbed. However, this is the first time, to the best of our knowledge, that it has been observed experimentally. If the number of water molecules desorbed to make place for one molecule of pyridine is larger than 4.4, a decrease in mass should be observed. When independent data on the dependence of surface coverage on potential is available, this number can be estimated. In the case of pyridine, the EQCM data best fit the adsorption results if each molecule is assumed to replace 10-12 water molecules. In the case of benzene a similar calculation could not be performed since there are no adsorption data collected under comparable experimental conditions. Nevertheless, a positive shift of frequency is observed, indicating that this number must be >4.4.